1. Field of the Invention
The present invention relates generally to implantable medical apparatus and, more specifically, relates to implantable electrodes and related apparatus.
2. Description of the Prior Art
The most popular electrode type for long-term attachment to the myocardial tissue is a structure having a helical electrode at its distal tip. This helical tip is screwed into the myocardium to establish chronic fixation and long-term electrical conductivity. Examples of such an electrode and insertion tools used in conjunction therewith may be found in U.S. Pat. No. 3,737,579 issued to Bolduc, U.S. Pat. No. 4,010,757 issued to Jula et al., and U.S. Pat. No. 4,207,903 issued to O'Neill, all assigned to the assignee of the present invention. These references teach the current state of the art relative to the insertion apparatus and techniques for implanting such chronic myocardial electrodes.
A problem with the techniques taught by the above-described references is in electrically mapping the myocardial surface. It is desirable during the implant procedure to find a location for installation of the electrode which provides optimal sensing and stimulation thresholds. The normal manner of doing this with the structures taught above is to press the helical electrode into contact with the epicardium and measure the sensed voltage. The disadvantage of this technique is that the helical electrode after chronic implantation will be screwed into the myocardium. Therefore, it will contact much different tissue than is contacted during the mapping procedure. The alternative of using this procedure is to screw the helical electrode into the myocardium to make mapping measurements. However this is considered far more traumatic to the myocardium then is justified.
A number of structures have been developed to enhance the mapping process. One such technique is taught by Barton et al. in U.S. Pat. No. 4,187,853. Barton et al. teach a probing needle 52 having a pointed distal end at 54 which is attached to the insertion tool. During the implantation procedure, point 54 of probe 52 is extended into the myocardium to make testing measurements. The probe is retracted into the lead body after the desired location has been found and the helical electrode permanently implanted using the standard procedure. This technique has as its major disadvantage, that the electrical path for conductivity during the mapping procedure does not involve the electrode to be chronically implanted. Therefore, the sensing and stimulation impedances observed during mapping will be substantially different from those to be experienced after chronic implantation.
An alternative technique is taught by Weiss in U.S. Pat. No. 4,235,246. As taught by Weiss, probing needle 38 is attached to the chronically implantable electrode. This techniques overcomes the problems found in the Barton et al method because it uses the electrically conductive path of the chronically implantable lead for mapping purposes. The Weiss technique, however, unduly complicates the chronically implantable electrode. That is to say, the retractable probing needle remains with the electrode after chronic implantation and, therefore, must itself be made highly biocompatible and must be of sufficient reliability not to contribute to the overall failure rate of the chronically implantable electrode.
The present invention uses a disposable exploring needle which uses the complete electrical path of the chronically implantable lead for the electrical mapping purposes, and yet does not impact the physical size or the characteristics of the electrode in its chronically implanted state.